The present invention relates to a system and method for determining the center of gravity of a loaded aircraft in a simple manner.
The location of the center of gravity of a loaded aircraft is a critical factor in determining whether the aircraft is safe for flight. Determining the location of the center of gravity (CG) is particularly important for cargo transport aircraft. This is because the loads carried by such aircraft may vary significantly from flight to flight in terms of load weight distribution. The CG for a given aircraft must be maintained within a certain range for a given load as specified by the aircraft manufacturer and/or the Federal Aviation Administration. One method for determining CG manually would be to determine the weight at a series of points along the aircraft""s length to calculate the moment developed about a given reference line. These moments are then summed and divided by the total weight of the aircraft and the load to determine the point at which the plane is balanced. This point is the aircraft CG. Aircraft CG is sometimes expressed in terms of percentage mean aerodynamic chord (% MAC).
Because this method, if done manually, would be tedious, time consuming, and expensive, a variety of systems and devices have been devised to determine aircraft CG. Many of these systems use strain gauges mounted in the oleo struts that support the aircraft. The amount of strain in the struts can be related to the force on the struts and therefore aircraft weight. Data concerning the weight at each of the aircraft wheels can be related to aircraft center gravity. Other systems have relied on pressure transducers in fluid communication with landing gear hydraulic systems. One of these requires a separate pump to cycle each oleo strut between a near fully extended position and a near fully retracted position while recording a series of pressure measurements. The pressure measurements are then related to weight on the strut.
These systems have not been without their problems, however. As these struts age due to the wear and tear of daily use, the strain gauge or pressure readings may become unreliable. This stems from friction in the landing gear struts and hydraulic system leaks. Moreover, the strain gauge systems may suffer from a high noise to signal ratio in the information provided by the strain gauges.
There remains a need then for a simple, low cost system for determining aircraft CG. The system should minimize the need for costly electronic components that must stand up to the environmental rigors to which aircraft landing gear are subjected. Desirably, such a system should be separable from the aircraft, should not require any type of expensive hydraulic system interface and should be simple to operate.
The present invention relates to a method, system and apparatus for determining the center of gravity of an aircraft, particularly, smaller aircraft not otherwise provided with equipment for such determination. By determining the vertical load on a known point on the longitudinal axis of the aircraft between the landing gear and either front or rear end of the aircraft (preferably the nose wheel in a tricycle wheel arrangement) necessary to maintain equilibrium, aircraft CG is determined. To determine the load (1) the nose wheel may be positioned on a load cell imbedded in the ground surface, (2) the nose wheel may be placed on a portable surface in which the load cell is mounted, or (3) there may be provided a simple apparatus for lifting the nose or tail from the ground (such as a lever, jack, or other device) and measuring the reactive load thus generated. By summing the moments about the reactive load point and then about the lever fulcrum, the nose wheel load can be related to aircraft CG. This process must take into account aircraft inclination to provide accurate results. An inclinometer mounted on or attached to the aircraft, provides the aircraft inclination.
The invention further relates to a method for determining the longitudinal center of gravity of an aircraft having a plurality of landing gear wheels. The method includes the steps of determining aircraft basic operating weight (empty weight plus crew and equipment) total aircraft weight, which is the basic operating weight (empty weight+crew+equipment), fuel load, and payload weight. The load at a selected point along the aircraft longitudinal axis so as to maintain equilibrium is then determined, while simultaneously determining the inclination of the aircraft. This load is then related to the aircraft total weight so as to determine the longitudinal position of the aircraft center of gravity.
Assuming the aircraft is substantially level and includes a nose wheel and a laterally spaced apart main landing gear, the load on the nose wheel can be related to the aircraft center of gravity location according to the relationship:
Lm=L/(1+(Pm/Pn)) 
where
L equals the horizontal distance between the nose wheel and the main landing gear;
Lm equals the horizontal distance between the center of gravity and the main landing gear;
Pm equals the load on the main landing gear; and
Pn equals the load on the nose wheel.
However, CG determination must oftentimes be made when an aircraft is not level, but inclined slightly where the aircraft is inclined at an angle of inclination and includes a nose wheel and a laterally spaced apart main landing gear, the load on the nose wheel can be related to the aircraft center of gravity location according to the relationship:                               L          m                ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                                            P                n                            ⁢                              xe2x80x83                            ⁢              L                                                      (                WT                )                            ⁢                              xe2x80x83                            ⁢                              cos                2                            ⁢                              xe2x80x83                            ⁢              θ                                ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                                    L              zm                        ⁢                          xe2x80x83                        ⁢            tan            ⁢                          xe2x80x83                        ⁢            θ                                              xe2x80x83            
where
WT equals total aircraft weight;
Pn equals aircraft nose wheel load;
xcex8 equals aircraft inclination;
L equals the horizontal distance between center of nose wheel and landing gear;
Lzm equals the vertical position of CG above the main landing gear wheel axle; and
Lm equals the horizontal distance between CG and main landing gear.
The invention further relates to a method for determining the longitudinal center of gravity of an aircraft having a nose wheel and laterally spaced apart main landing gear wheels including measuring the aircraft inclination; determining total aircraft weight; measuring the horizontal distance between the center of the nose wheel and the main landing gear wheels; measuring the load on the nose wheel; determining the angle of inclination of the aircraft at the time the load is measured and relating the nose wheel load to aircraft center of gravity according to the relationship described above.
The aircraft nose wheel load may be measured by placing the nose wheel on a load cell imbedded in the ground or on a portable platform in which the load cell is mounted. The inclination of the airplane can be measured by an inclinometer in the airplane or attached thereto, or otherwise.
In another embodiment the system includes a lever apparatus for determining the load on one of the plurality of aircraft wheels including:
i. a frame having a forward end and an aft end, and adapted for detachable engagement with one of the plurality of aircraft wheels;
ii. a fulcrum point about which the lever apparatus raises the aircraft wheel;
iii. a load cell mounted on the frame and adapted to generate a signal indicative of the reactive force generated on the frame as the wheel is raised.
Each system further includes a processor for receiving flight data and the load cell signal, determining wheel load, determining aircraft inclination, and relating wheel load, aircraft inclination and flight data to aircraft center of gravity.
The present invention further relates to an apparatus for determining the load on an aircraft wheel having a wheel axle including a lever apparatus having a forward end and an aft end, the aft end adapted for detachable engagement with an aircraft wheel axle. The lever apparatus in includes a fulcrum point spaced laterally apart from the wheel axle. A load cell is mounted on the frame so as to generate a signal indicative of the reactive force generated on the frame at a point between the forward end and the aft end as the wheel is raised responsive to a force applied to the forward end. The apparatus further includes a wheel load processor for receiving the load cell signal and determining aircraft wheel load.
In an alternative embodiment, the apparatus may further include a pivot arm having a first end and a second end, the first end pivotally attached to the lever apparatus at a pivot point located between the forward end and the aft end of the apparatus. The apparatus and the pivot arm thus define a lever for raising the wheel so that the load cell is engaged by the pivot arm.
One aspect of the present invention is to provide a technique for determining aircraft CG that may be operated by one person and that may be stowed on an aircraft when not in use.
Another aspect of the present invention is the to provide a technique for determining aircraft CG that eliminates the need for multiple, landing gear-mounted load cells.
Still another aspect of the present invention is to provide a method for determining aircraft CG that accounts for aircraft inclination.
A further aspect of the present invention is the provision of a tow bar that may also serve duty as a lever apparatus for determining aircraft CG.
These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiments when considered in conjunction with the drawings.